Environment Variable Diagnostics

Description

Diagnose and fix environment variable and configuration problems - .env load order in Next.js, Vite and dotenv, validating env at startup with Zod or a small validator so the app fails fast, values that are wrong rather than missing (boolean and number strings, whitespace, unquoted spaces, multiline private keys), auditing which variables the code uses, "works locally, fails in CI/production" across Vercel, Docker, Compose and Kubernetes, public prefixes (NEXT_PUBLIC_, VITE_, EXPO_PUBLIC_) and what leaks to the browser, Docker ARG vs ENV and secrets, and keeping .env.example authoritative. Use whenever a value is undefined or wrong, the app works locally but not in CI, Docker or production, the user edits .env files, asks why a feature flag is always on, adds a new environment variable, or exposes config to client code.